High contrast electro-optic liquid crystal camera iris including liquid crystal material mixed with a dye to improve achromatic performance

ABSTRACT

A high-contrast electro-optic liquid crystal camera iris provides angle independent transmission for uniform gray shades. The liquid crystal iris comprises a combination of first and second liquid crystal devices arranged in optical series and positioned between optical polarizers. The director field of the second liquid crystal device is a mirror image of the director field of the first liquid crystal device, and the first and second liquid crystal devices are placed together so that the azimuthal directions of the surface-contacting directors are in parallel alignment at the adjoining or confronting surfaces of the substrates of the first and second liquid crystal devices. The liquid crystal iris provides, therefore, less angular variation of intermediate transmittances compared with that provided by prior art liquid crystal irises.

RELATED APPLICATIONS

This is a division of U.S. patent application Ser. No. 15/024,303, filedMar. 24, 2016, now U.S. patent Ser. No. 10/012,884, which is a 371 ofInternational Application No. PCT/US2014/056553, filed Sep. 19, 2014,which claims benefit of U.S. Provisional Patent Application No.61/976,408, filed Apr. 7, 2014, and is a continuation-in-part of U.S.patent application Ser. No. 14/034,283, filed Sep. 23, 2013, now U.S.Pat. No. 8,891,042.

COPYRIGHT NOTICE

© 2018 LC-TEC Displays AB. A portion of the disclosure of this patentdocument contains material that is subject to copyright protection. Thecopyright owner has no objection to the facsimile reproduction by anyoneof the patent document or the patent disclosure, as it appears in thePatent and Trademark Office patent file or records, but otherwisereserves all copyright rights whatsoever. 37 CFR § 1.71(d).

TECHNICAL FIELD

This disclosure relates to a liquid crystal iris for an image recordingdevice such as a camera and, in particular, for a miniature camera of atype installed in a smart phone.

BACKGROUND INFORMATION

A conventional camera uses a mechanical iris diaphragm to control theamount of light reaching the recording medium, such as film or acharge-coupled device (CCD) light sensor array. The mechanical iris iscomplex device that is ill-suited for many miniature cameraapplications, such as those found in smart phones and other hand-helddevices. Many electro-optical alternatives to the mechanical iris havebeen proposed such as the guest-host liquid crystal display (U.S. Pat.No. 4,774,537 and U.S. Patent Application Pub. No. US 2012/0242924),twisted nematic (TN) liquid crystal display (U.S. Patent ApplicationPub. No. US 2008/0084498), electrophoretic display (U.S. Pat. No.7,859,741), digital micro-mirror display (European Patent ApplicationNo. EP1001619 A1), and electrowetting display (U.S. Pat. No. 7,508,566).All of these alternatives can control the amount of light admitted tothe recording medium, and if the electrode structures are patterned inan arrangement of concentric rings to enable changing the aperture,these alternative approaches can also adjust the depth of field. Such apatterned ring arrangement of electrode structures is described indetail, for example, in U.S. Patent Application Pub. No. US2008/0084498.

Using liquid crystals for an electro-optical iris is particularlyattractive since liquid crystal devices (LCDs) are a mature mainstreamtechnology. As with the mechanical iris, to control light effectively,the liquid crystal iris must be capable of providing not only a highcontrast ratio, but also a uniform transmittance at intermediate graylevels over the range of light input angles that are typically found inminiature camera optics. Angular dependence of the transmittance of theiris would cause a nonuniform exposure over the light-sensitive area ofthe recording medium. Prior art liquid crystal iris designs do notsatisfy these requirements, which shortcomings are reasons why liquidcrystal irises have not yet found widespread commercial application.

Simulations show that a TN device used as a camera iris, as proposed inU.S. Patent Application Pub. No. US 2008/0084498, provides a very highcontrast ratio, but the transmittance at intermediate gray levels isappreciably nonuniform over a range of angles of incident light. FIG. 1shows a simulated electro-optic curve of a prior art TN iris deviceunder conditions of normally incident white light. This curve issimulated by use of Display Modeling System (DIMOS) software availablefrom Autronic-Melchers GmbH, Karlsruhe, Germany, for the liquid crystalmixture MLC-7030 available from Merck GmbH, Darmstadt, Germany. MLC-7030has a positive dielectric constant anisotropy of 3.8 and a birefringenceof Δn=0.1126 at the 550 nm design wavelength. The cell gap is chosen as4.23 μm to satisfy the “first minimum” condition Δn·d/λ=0.866 to providemaximum throughput at the design wavelength. Using the data of FIG. 1,the normalized transmitted luminance is 50% at 2.81V, and the normalizedtransmitted luminance is 0.1% at 5.25V, resulting in a contrast ratio of1,000.

FIG. 2 shows, for this simulation, the angular variation of thenormalized transmitted luminance of the prior art TN iris underapplication of a drive voltage of 2.81V for 50% transmitted luminance atnormal incidence. These data are conveniently presented by aniso-transmitted luminance polar contour diagram, in which the contoursare lines of constant normalized transmitted luminance. The center ofthe diagram represents normal incidence, where the normalizedtransmitted luminance is 50% and the periphery of the diagram representsincident light at the polar angle of 40°. The azimuthal angle of theincident light is represented in the circular direction from 0° to 360°.FIG. 2 shows that the normalized transmitted luminance varies from about9% to about 82% over incident angles extending out to 20°. Although thisTN device can achieve a high contrast ratio, the strong angularvariation of intermediate transmittances of this TN device makes itunsatisfactory for use as a camera iris.

The prior art dual-cell guest-host iris represented in FIGS. 6A and 6Bof U.S. Patent Application Pub. No. US 2012/0242924, consists of twohomogeneously aligned guest-host cells placed in optical series andoriented with their surface alignment directions at 90°. FIG. 3 shows asimulated electro-optic curve for this prior art iris with 5 μm cellgaps filled with the liquid crystal mixture MLC-7030, to which is addedan achromatic organic dye mixture with a dichroic ratio of 6.2.Simulations show that, even at 12V, the contrast ratio reaches only 4.1.FIG. 4 shows a simulated iso-transmitted luminance polar contour diagramof the normalized transmitted luminance under application of 3.1V for50% normalized transmitted luminance at normal incidence. In FIG. 4, thenormalized transmitted luminance varies from about 42% to 60% over arange of incident angles extending out to polar angles of 20°. FIG. 4exhibits somewhat less angular variation of intermediate transmittancescompared with the angular variation of the TN iris simulated in FIG. 2,but the low contrast ratio of the dual-cell guest-host iris makes itunsatisfactory for use as a camera iris.

SUMMARY OF THE DISCLOSURE

The disclosed liquid crystal electro-optic iris overcomes the shortfallsof prior art liquid crystal irises by providing a high contrast ratiowith little angular variation of intermediate transmitted luminances.The disclosed liquid crystal iris comprises a combination of first andsecond liquid crystal cells or devices arranged in optical series andpositioned between optical polarizers. The director field of the secondliquid crystal device is a mirror image of the director field of thefirst liquid crystal device, and the two liquid crystal devices areplaced together so that the azimuthal directions of thesurface-contacting directors are in parallel alignment at the adjoiningor confronting surfaces of the substrates of the two liquid crystaldevices. This arrangement of the first and second liquid crystal devicesis notably different from that of prior art dual-cell assemblies, inwhich the azimuthal directions of the surface-contacting directors ofthe adjacent substrates are in orthogonal alignment to each other.

A qualitative explanation for the above-described arrangement of thefirst and second liquid crystal devices of the present invention is asfollows. For example, in the first liquid crystal device, intermediatetransmitted luminances can exhibit considerable angular variation whenthe propagation direction of the light in the liquid crystal layer isapproximately along the direction of the tilted director located in themiddle of the liquid crystal layer. But, as the light passes through thesecond liquid crystal device, the angular variation of the combinedtransmittance is dramatically reduced because the tilted directorlocated in the middle of the second liquid crystal layer is morebroadside to the light propagation direction. Placing the two liquidcrystal devices together in the arrangement described results,therefore, in light that passes through the first liquid crystal devicein a “bad” direction and passes through the second liquid crystal devicein a “good” direction, and vice versa. The disclosed dual-cell liquidcrystal iris provides, therefore, less angular variation of intermediatetransmittances compared with that provided by prior art liquid crystalirises.

In a first embodiment, the liquid crystal devices are non-twisted,electrically controlled birefringence (ECB) cells, in which the nematicliquid crystal can be of a type with positive dielectric constantanisotropy or with negative dielectric constant anisotropy. For the caseof positive dielectric anisotropy, the surface-contacting directors makesmall pretilt angles from the plane of the liquid crystal layer(homogeneous alignment) and, for the case of negative dielectricanisotropy, the surface-contacting directors make small pretilt anglesfrom the direction of the normal to the liquid crystal layer(quasi-homeotropic alignment). The cell gap of each liquid crystaldevice is chosen to provide a continuous, voltage-dependent phase shiftchange up to 90° between the ordinary and extraordinary polarizationcomponents of light passing through the device. The desired phase shiftchange is accomplished by placing the two liquid crystal devicestogether so that the azimuthal directions of the surface-contactingdirectors of the adjoining or confronting surfaces of the substrates ofthe two liquid crystal devices are in parallel alignment. The 90° phaseshift change imparted by each of the liquid crystal devices adds up to a180° phase shift change for the combination, which is equivalent to a90° rotation of linearly polarized light to provide maximum lighttransmittance when the first and second liquid crystal devices areplaced between orthogonally aligned polarizers.

In a second embodiment, the liquid crystal has a positive dielectricanisotropy, and the liquid crystal layer twist angles of the two liquidcrystal devices are substantially 60° but of opposite twist sensebecause the director field of the second liquid crystal device is amirror image of the director field of the first liquid crystal device.Furthermore, in this second embodiment, the product of the cell gap, d,times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.629, where λ is the design wavelength. Thetwo liquid crystal devices are placed together so that the azimuthaldirections of the surface-contacting directors of the adjoining orconfronting surfaces of the substrates of the two liquid crystal devicesare in parallel alignment. In this second embodiment, the inputpolarization direction is set to approximately bisect the angulardistance between azimuthal directions of the surface-contactingdirectors at the inner substrate surfaces of the first liquid crystaldevice.

When no voltage is applied to the liquid crystal devices of the secondembodiment, the light exiting the first liquid crystal device iscircularly polarized at the design wavelength and the light exiting thesecond liquid crystal device is linearly polarized, but rotated by 90°from the input linear polarization direction. When the liquid crystaldevices of this arrangement are activated with optimum high drivevoltage, the light exiting the first liquid crystal device is linearlypolarized and the light exiting the second liquid crystal device islinearly polarized in the same direction as the input linearpolarization direction. Placing the second embodiment betweenorthogonally aligned polarizers provides maximum light transmittancewhen no voltage is applied to the first and second liquid crystaldevices and essentially zero transmittance when a high voltage isapplied to the first and second liquid crystal devices, thereby assuringmaximum throughput and a high contrast ratio. Intermediate gray levelsare obtained by applying voltages intermediate between 0V and the highdrive voltage.

In a third embodiment, the liquid crystal has a positive dielectricanisotropy, and the liquid crystal layer twist angles of the two liquidcrystal devices are substantially 90° but of opposite twist sensebecause the director field of the second liquid crystal device is amirror image of the director field of the first liquid crystal device.Furthermore, in this third embodiment the product of the cell gap, d,times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.447, where λ is the design wavelength. Thetwo liquid crystal devices are placed together so that the azimuthaldirections of the surface-contacting directors of the adjoining orconfronting surfaces of the substrates of the two liquid crystal devicesare in parallel alignment. In this third embodiment, the inputpolarization direction is set to approximately 20° with respect to theazimuthal direction of the surface-contacting directors at the inputsurface of the first liquid crystal device.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simulated normalized transmitted luminance electro-opticcurve of a prior art iris comprising a TN liquid crystal device.

FIG. 2 shows a normalized iso-transmitted luminance polar contourdiagram of the prior art camera iris of FIG. 1, comprising a single TNliquid crystal device with a drive voltage adjusted to give 50%normalized transmitted luminance for normally incident light.

FIG. 3 is a simulated normalized transmitted luminance electro-opticcurve of a prior art camera iris comprising two orthogonally alignedguest-host liquid crystal devices.

FIG. 4 shows a simulated normalized iso-transmitted luminance polarcontour diagram of the prior art camera iris of FIG. 3, comprising twoorthogonally aligned guest-host liquid crystal devices with a drivevoltage adjusted to give 50% normalized transmitted luminance fornormally incident light.

FIG. 5 is a diagrammatic exploded view of a first implementation of afirst embodiment of the disclosed electro-optic liquid crystal camerairis comprising first and second homogeneously aligned ECB devices, inwhich the director field of the second liquid crystal device is a mirrorimage of the director field of the first liquid crystal device, and inwhich the first and second liquid crystal devices are placed together sothat the azimuthal directions of the surface-contacting directors of theadjoining or confronting surfaces of the substrates of the two liquidcrystal devices are in parallel alignment.

FIG. 6 is a normalized transmitted luminance electro-optic curve of thefirst embodiment of FIG. 5, showing the normalized transmitted luminanceas a function of applied voltage.

FIGS. 7A, 7B, and 7C show normalized iso-transmitted luminance polarcontour diagrams of the camera iris according the first embodimentdepicted in FIG. 5, with the drive voltage adjusted to give,respectively, 75%, 50%, and 25% normalized transmitted luminance fornormally incident light.

FIG. 8A shows a measured normalized transmitted luminance electro-opticcurve according to the first embodiment depicted in FIG. 5.

FIGS. 8B, 8C, and 8D show measured iso-transmitted luminance polarcontour diagrams according to the first embodiment depicted in FIG. 5,taken at, respectively, 1.41V with 75% normalized gray level at normalincidence, 1.74V with 50% normalized gray level at normal incidence, and2.22V with 25% normalized gray level at normal incidence.

FIG. 9 is a diagrammatic exploded view of a second implementation of afirst embodiment of the disclosed electro-optic liquid crystal camerairis comprising first and second vertically aligned nematic (VAN)devices, in which the director field of the second liquid crystal deviceis a mirror image of the director field of the first liquid crystaldevice, and in which the first and second liquid crystal devices areplaced together so that the azimuthal directions of thesurface-contacting directors of the adjoining or confronting surfaces ofthe substrates of the two liquid crystal devices are in parallelalignment.

FIG. 10 is the measured transmitted luminance electro-optic curve of anexample of the first embodiment of FIG. 9, showing the transmittedluminance as a function of applied voltage. Data points indicating unitf-stop settings from f/2 to f/12 are superimposed on the curve.

FIGS. 11A, 11B, and 11C show measured iso-transmitted luminance polarcontour diagrams of a liquid crystal iris comprising a single, prior-artVAN liquid crystal device measured at f-stop settings of f/2, f/6, andf/12.

FIGS. 12A, 12B, and 12C show measured iso-transmitted luminance polarcontour diagrams of a liquid crystal iris of FIG. 9 comprising two VANliquid crystal devices but no retarder film, measured at f-stop settingsof f/2, f/6, and f/12.

FIGS. 13A, 13B, and 13C show measured iso-transmitted luminance polarcontour diagrams of a liquid crystal iris of the second implementationof the first embodiment of FIG. 9 comprising two VAN liquid crystaldevices and a biaxial retarder, measured at f-stop settings of f/2, f/6,and f/12.

FIG. 14 shows the color coordinates for different f-stop settings in the1976 CIE (u′, v′) uniform color space of the liquid crystal iris of thesecond implementation of the first embodiment of the liquid crystalcamera iris. The three different V-shaped curves correspond to threedifferent concentrations of dichroic dye doping.

FIG. 15 shows the measured ordinary α_(O) and extraordinary α_(E)absorption coefficients of 1 wt. % solution of the yellow Nagase dyeG-470.

FIG. 16 shows the measured ordinary α_(O) and extraordinary α_(E)absorption coefficients of 1 wt. % solution of the magenta Nagase dyeG-241.

FIG. 17 is a block diagram of the disclosed liquid crystal camera iristhat incorporates two color trimming devices into the liquid crystaliris to minimize color shifts between f-stop settings and to make theiris more achromatic.

FIG. 18 compares the color coordinates for unit f-stop settings betweenf/2 and f/8 in the 1976 CIE (u′, v′) uniform color space of the liquidcrystal iris of the second implementation of the first embodiment of theinvention with and without the use of a first color trimming device.

FIG. 19 shows the adjustment of the drive voltage required to maintainconstant f-stop settings of f/2, f/6, and f/10 for the secondimplementation of the first embodiment of FIG. 9.

FIG. 20 is a simulated normalized transmitted luminance electro-opticcurve of a second embodiment of the disclosed electro-optic liquidcrystal camera iris, showing the normalized transmitted luminance as afunction of applied voltage.

FIG. 21 shows a simulated normalized iso-transmitted luminance polarcontour diagram of the camera iris according the second embodiment, withthe drive voltage adjusted to give 50% normalized transmitted luminancefor normally incident light.

FIG. 22 is a simulated normalized transmitted luminance electro-opticcurve of a third embodiment of the disclosed electro-optic liquidcrystal camera iris, showing the normalized transmitted luminance as afunction of the applied voltage.

FIG. 23 shows the normalized iso-transmitted luminance polar contourdiagram of the camera iris according the third embodiment, with thedrive voltage adjusted to give 50% normalized transmitted luminance fornormally incident light.

FIG. 24 is a block diagram showing one example of a camera moduleimplemented with the disclosed liquid crystal iris.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 5 is a simplified diagram of a liquid crystal iris 10 configured asa first implementation of a first embodiment of the disclosedelectro-optic liquid crystal camera iris. Liquid crystal iris 10comprises a first ECB liquid crystal device 12 and a second ECB liquidcrystal device 14, each of which contains liquid crystal having apositive dielectric anisotropy. For simplicity, index matching coatingsof each of ECB liquid crystal devices 12 and 14 are omitted from thediagram. ECB liquid crystal devices 12 and 14 have, respectively, adirector field 16 composed of liquid crystal directors 18 and a directorfield 20 composed of liquid crystal directors 22. Each of directorfields 16 and 20 is shown in FIG. 5 at an intermediate drive voltage, V.Director field 20 of ECB liquid crystal device 14 is a mirror image ofdirector field 16 of ECB liquid crystal device 12. In other words,liquid crystal directors 22 in director field 20 are reversely arrangedin comparison to corresponding liquid crystal directors 18 in directorfield 16.

ECB liquid crystal device 12 has a spaced-apart pair of first electrodestructures that include substrate plates 24 ₁ and 24 ₂. An opticallytransparent electrode 26 ₁ formed on substrate plate 24 ₁ constitutes,for one first electrode structure of the pair, an interior surface onwhich is formed an alignment layer 28 ₁. An optically transparentelectrode 26 ₂ formed on substrate plate 24 ₂ constitutes, for the otherfirst electrode structure of the pair, an interior surface on which isformed an alignment layer 28 ₂. Alignment layers 28 ₁ and 28 ₂ haverespective alignment surfaces 30 ₁ and 30 ₂.

ECB liquid crystal device 14 has a spaced-apart pair of second electrodestructures that include substrate plates 32 ₁ and 32 ₂. An opticallytransparent electrode 34 ₁ formed on substrate plate 32 ₁ constitutes,for one second electrode structure of the pair, an interior surface onwhich is formed an alignment layer 36 ₁. An optically transparentelectrode 34 ₂ formed on substrate plate 32 ₂ constitutes, for the othersecond electrode structure of the pair, an interior surface on which isformed an alignment layer 36 ₂. Alignment layers 36 ₁ and 36 ₂ haverespective alignment surfaces 38 ₁ and 38 ₂.

Surface-contacting directors 18 c and 22 c make angles α with theirrespective alignment surfaces 30 ₁, 30 ₂ and 38 ₁, 38 ₂. Azimuthaldirections of the surface-contacting directors are indicated by arrows.Specifically, arrows 40 indicate the azimuthal direction ofsurface-contacting directors 18 c and 22 c at alignment surfaces 30 ₁and 38 ₂, respectively; and arrows 42 indicate the azimuthal directionof surface-contacting directors 18 c and 22 c at alignment surfaces 30 ₂and 38 ₁. Arrows 42 are parallel at the adjoining or confrontingsurfaces of substrate plates 24 ₂ and 32 ₁ of ECB liquid crystal devices12 and 14, respectively. To obtain high contrast ratios, one or moreexternal retarders are included in liquid crystal iris 10 to compensatefor the combined residual retardation of ECB liquid crystal devices 12and 14. The slow axis of the retarder is set in generally perpendicularalignment to the azimuthal direction of the surface-contacting directorsof the alignment layer to which the retarder is in adjacent position,although other retarder axis orientations are also possible. Theretarder or retarders can be placed at locations 44 ₁, 44 ₂, and 44 ₃,as indicated in FIG. 5. ECB liquid crystal devices 12 and 14 arepositioned between linear polarizers 46 and 48, with their transmissionaxes orthogonally aligned. Incoming light 50 is incident on polarizer46.

FIG. 6 shows a simulated electro-optic curve of an example of the firstimplementation of the first embodiment represented by liquid crystaliris 10. The liquid crystal MLC-7030 is used in the simulation, and thecell gap of each of ECB liquid crystal devices 12 and 14 is set at 1.50μm. The pretilt angle α is 3°. To obtain a high contrast ratio, 31.4 nmpolycarbonate retarders are placed at locations 44 ₂ and 44 ₃, withtheir slow axes set in perpendicular alignment to azimuthal direction 40of the surface-contacting directors 18 c and 22 c of ECB liquid crystaldevices 12 and 14, respectively. In this orientation, the two retardersfully compensate the combined residual retardation of ECB liquid crystaldevices 12 and 14 at an applied voltage of 6.51V. The combination of ECBliquid crystal devices 12 and 14 and associated retarders at locations44 ₂ and 44 ₃ is placed between orthogonally aligned polarizers 46 and48 such that azimuthal directions 40 of surface-contacting directors 18c and 22 c of the respective ECB liquid crystal devices 12 and 14 make a45° angle with the polarization direction of light incident on a lightinput face 52 of substrate plate 24 ₁. The normalized transmittedluminance is 50% for an applied voltage of 2.95V, and the normalizedtransmitted luminances at 6.16V is 0.1%, thereby resulting in a contrastratio of 1,000.

FIGS. 7A, 7B, and 7C show the angular dependence of the normalizedtransmitted luminance under application of three different drivevoltages, V. Drive voltages of 2.55V, 2.95V, and 3.47V, give,respectively, 75%, 50%, and 25% transmitted luminance at normalincidence. As in FIG. 2, these data are presented in FIGS. 7A, 7B, and7C in the form of normalized iso-transmitted luminance polar contourdiagrams. Comparing FIG. 7B with FIG. 2 for the prior art TN iris, it isapparent that there is remarkable improvement in the uniformity of theangular dependence of the 50% gray level. FIGS. 7A and 7C show gooduniformity of angular dependence of the 75% gray level and of the 25%gray level, respectively. Liquid crystal iris 10 of the first embodimentis eminently suitable for use as a camera iris because it can achieve ahigh contrast ratio and exhibits capability to maintain a uniform graylevel over a wide range of light input angles.

FIG. 8A shows a measured normalized transmitted luminance electro-opticcurve at normal incidence for an example of liquid crystal iris 10 ofthe first implementation of the first embodiment. The liquid crystalused in ECB liquid crystal devices 12 and 14 for this measurement has abirefringence of 0.099 at 589 nm and 20° C., and the cell gap of each ofECB liquid crystal devices 12 and 14 is set at 1.4 μm. A first 15 nmretarder film is placed in retarder location 44 ₂, at light input face52 of substrate plate 24 ₁ of ECB liquid crystal device 12. The slowaxis of the first 15 nm retarder is set perpendicular to azimuthaldirection 40 of surface-contacting directors 18 c at light input face52. A second 15 nm retarder film is placed in retarder location 44 ₃, ata light exit face 54 of substrate plate 32 ₂ of ECB liquid crystaldevice 14. The slow axis of the second 15 nm retarder is setperpendicular to azimuthal direction 40 of surface-contacting directors22 c at light exit face 54. The combination of ECB liquid crystaldevices 12 and 14 and associated retarders positioned in locations 44 ₂and 44 ₃ is placed between orthogonally aligned polarizers 46 and 48,such that azimuthal direction 40 of surface-contacting directors 18 cand 22 c of the respective ECB liquid crystal devices 12 and 14 make a45° angle with the polarization direction of the incident light. Forapplied voltages of 1.41V, 1.74V, and 2.22V, the normalized transmittedluminance at normal incidence is 75%, 50%, and 25%, respectively.

FIGS. 8B, 8C, and 8D show measured iso-transmitted luminance polarcontour diagrams of the above example of liquid crystal iris 10 of thefirst embodiment for applied voltages of 1.41V, 1.74V, and 2.22V, forwhich the normalized transmitted luminance at normal incidence is 75%,50%, and 25%, respectively. The central portions of these diagramsexhibit only a weak angular dependence of the transmission, therebyverifying the simulated results and showing the suitability of liquidcrystal iris 10 of the first implementation of the first embodiment fora camera iris application.

FIG. 9 is a simplified diagram of a liquid crystal iris 10′″ configuredas a second implementation of the first embodiment of the disclosedelectro-optic liquid crystal camera iris. The second implementation isone in which the liquid crystal has a negative dielectric anisotropy andthe surface contacting directors make small pretilt angles from thedirection normal to the liquid crystal layer. The second implementationalso uses two ECB liquid crystal devices, which for the case of negativedielectric anisotropy liquid crystals are more commonly referred to asvertically aligned nematic, or VAN, liquid crystal devices.

The construction of the first embodiment using VAN liquid crystaldevices is the same as that of liquid crystal iris 10 shown in FIG. 5for the first implementation of the first embodiment that uses positivedielectric anisotropy liquid crystals, except as described below. Thedirector fields of the liquid crystal devices in the secondimplementation differ from the director fields 16 and 20 of ECB liquidcrystal devices 12 and 14 of the first implementation of the firstembodiment. The liquid crystal devices and their associated directorfields of the second implementation shown in FIG. 9 are, therefore,indicated by corresponding reference numerals followed by triple primes(′″).

Liquid crystal iris 10′″ comprises a first VAN liquid crystal device12′″ and a second VAN liquid crystal device 14′″, each of which containsliquid crystal having a negative dielectric anisotropy. For simplicity,index matching coatings of each of VAN liquid crystal devices 12′″ and14′″ are omitted from the diagram. VAN liquid crystal devices 12′″ and14′″ have, respectively, a director field 16′″ composed of liquidcrystal directors 18′″ and a director field 20′″ composed of liquidcrystal directors 22′″. Each of director fields 16′″ and 20′″ is shownin at an intermediate drive voltage, V. Director field 20′″ of VANliquid crystal device 14′″ is a mirror image of director field 16′″ ofVAN liquid crystal device 12′″. In other words, liquid crystal directors22′″ in director field 20′″ are reversely arranged in comparison tocorresponding liquid crystal directors 18′″ in director field 16′″.

VAN liquid crystal device 12′″ has a spaced-apart pair of firstelectrode structures that include substrate plates 24 ₁′″ and 24 ₂′″. Anoptically transparent electrode 26 ₁′″ formed on substrate plate 24 ₁′″constitutes, for one first electrode structure of the pair, an interiorsurface on which is formed an alignment layer 28 ₁′″. An opticallytransparent electrode 26 ₂′″ formed on substrate plate 24 ₂′″constitutes, for the other first electrode structure of the pair, aninterior surface on which is formed an alignment layer 28 ₂′″. Alignmentlayers 28 ₁′″ and 28 ₂′″ have respective alignment surfaces 30 ₁′″ and30 ₂′″.

VAN liquid crystal device 14′″ has a spaced-apart pair of secondelectrode structures that include substrate plates 32 ₁′″ and 32 ₂′″. Anoptically transparent electrode 34 ₁′″ formed on substrate plate 32 ₁′″constitutes, for one second electrode structure of the pair, an interiorsurface on which is formed an alignment layer 36 ₁′″. An opticallytransparent electrode 34 ₂′″ formed on substrate plate 32 ₂′″constitutes, for the other second electrode structure of the pair, aninterior surface on which is formed an alignment layer 36 ₂′″. Alignmentlayers 36 ₁′″ and 36 ₂′″ have respective alignment surfaces 38 ₁′″ and38 ₂′″.

Surface-contacting directors 18 c′″ and 22 c′″ make angles α with theirrespective alignment surfaces 30 ₁′″, 30 ₂′″ and 38 ₁′″, 38 ₂′″.Azimuthal directions of the surface-contacting directors are indicatedby arrows. Specifically, arrows 40′″ indicate the azimuthal direction ofsurface-contacting directors 18 c′″ and 22 c′″ at alignment surfaces 30₁′″ and 38 ₂′″, respectively; and arrows 42′″ indicate the azimuthaldirection of surface-contacting directors 18 c′″ and 22 c′″ at alignmentsurfaces 30 ₂′″ and 38 ₁′″. Arrows 42′″ are parallel at the adjoining orconfronting surfaces of substrate plates 24 ₂′″ and 32 ₁′″ of VAN liquidcrystal devices 12′″ and 14′″, respectively.

The transmission direction of polarizer 46′″ is set at 45°, and thetransmission direction of analyzer 48′″ is set at −45° with respect tothe azimuthal directions 40′″ and 42′″ of the surface contactingdirectors. Retarder film 44 ₃′″ placed at a location between light exitface 54′″ and polarizer 48′″ is the only retarder film present in thesecond implementation. Unlike the positive dielectric anisotropy liquidcrystal devices of the first implementation shown in FIG. 5, the VANliquid crystal devices of FIG. 9 have comparatively low residualretardances and generally do not require retardation films to compensatefor such low values, although such retardation films could be added. Thepurpose of retarder film 44 ₃′″ is to improve the angular uniformity ofthe light transmitted by iris 10′″. Retarder film 44 ₃′″ is a biaxialretarder having an in-plane retardation R_(o) and an out-of planeretardation R_(th), with the in-plane slow axis oriented parallel to thetransmission axis of the adjacent polarizer 48′″.

FIG. 10 shows a transmitted luminance electro-optic curve measured atnormal incidence for the second implementation represented by liquidcrystal iris 10′″ of FIG. 9. In this example, the liquid crystal has abirefringence of 0.08 and a dielectric anisotropy of −4.4, and the cellgap of each VAN liquid crystal device 12′″ and 14′″ is 2.40 μm.Polarizers 46′″ and 48′″ are of the high durability type with a 39%transmission in unpolarized light and have protective TAC layers, eachwith out-of-plane retardations of approximately −40 nm. The biaxialretarder has 55 nm of in-plane retardation (R_(o)) and 220 nm ofout-of-plane retardation (R_(th)). The points on the transmittedluminance electro-optic curve are transmitted luminance valuescorresponding to camera f-stop settings. A transmitted luminance of 25%,for example, corresponds to f/2, which is shown by the pointsuperimposed at the right end of the curve. With each increasing f-stopsetting, the transmitted luminance decreases by a factor of 2. For f/6,the transmitted luminance is 1.5625%; and for f/12, the point at theleft end of the curve, the transmitted luminance is 0.02441%.Intermediate points on the curve correspond to unit f-stop settingsbetween f/2 and f/12.

FIGS. 11A, 11B, and 11C show the angular dependence of the transmittedluminance at the f-stop settings of f/2, f/6, and f/12 for the prior artcase of liquid crystal iris comprising a single VAN liquid crystaldevice. The angular data is presented in the form of iso-transmittedluminance polar contour diagrams. The uniformity of the angulardependence of the transmitted luminance at f/6 and f/12 is poor. FIGS.12A, 12B, and 12C show the angular dependence of the transmittedluminance at the f-stop settings of f/2, f/6, and f/12 for the case inwhich the liquid crystal iris 10′″ comprises two VAN liquid crystaldevices assembled as shown in FIG. 9, but without retarder film 44 ₃′″.The uniformity of the angular dependence of the transmitted luminance atf/6 is greatly improved, but the uniformity at f/12 is still poor. FIGS.13A, 13B, and 13C show the angular dependence of the transmittedluminance at the f-stop settings of f/2, f/6, and f/12 for the case inwhich the liquid crystal iris comprises two VAN liquid crystal devicesassembled as shown in FIG. 9 and including biaxial retarder 44 ₃′″described earlier. Comparison of FIG. 13C to FIG. 12C reveals that theuniformity of the angular dependence of the transmitted luminance atf/12 has been greatly improved by adding biaxial retarder 44 ₃′″.

Of course, many other configurations of the second implementation of thefirst embodiment are also possible. The biaxial retarder could be placedon top of the first VAN liquid crystal device 12′″, instead of at thebottom of the second VAN liquid crystal device 14′″; or a biaxialretarder could be placed at both top and bottom locations. The biaxialretarder could also be replaced by an A plate retarder and a negative Cplate retarder, either in combination or separated, one on each side ofthe stack of VAN liquid crystal devices 12′″ and 14′″. A negative Cplate retarder could also be placed between VAN liquid crystal devices12′″ and 14′″. Similarly, a biaxial retarder could also be added to thefirst implementation of the first embodiment, in which the liquidcrystal has a positive dielectric anisotropy.

Ideally, the liquid crystal iris should have achromatic performance overthe full range of f-stop settings, i.e., it should not introduce anycoloration of its own. Measurements of the performance of liquid crystaliris 10 of the second implementation of the first embodiment with VANliquid crystal devices 10′″ and 12′″ show a small f-stop dependent colorshift. This is quantitatively indicated by the measured “no dye” curvein FIG. 14, where the (u′,v′) color coordinates at f-stop settingsranging from f/3 to f/10 are indicated on the 1976 CIE uniform colorspace. The V-shaped curve, called the f-stop color gamut, is locatedsome distance away from the equal energy white point E at(u′,v′)=(0.2105, 0.4737). By adding a mixture of dichroic dyes to theliquid crystal it is possible not only to reduce the overall size of thef-stop color gamut, but also to move the f-stop color gamut in adirection closer to the white point. This is illustrated in FIG. 14 bythe dashed curve “0.6% dye mix,” which shifts the V-shaped curve part ofthe way toward the white point and by the dotted curve “1.2% dye mix,”which brings the V-shaped curve in close proximity to the white point.

The 0.6% dye mix comprises the nematic liquid crystal to which is added0.3 wt. % of the yellow dye G-470 and 0.3 wt. % of the magenta dyeG-241. The 1.2% dye mix comprises the nematic liquid crystal with 0.6wt. % of each of these dyes. The G-470 and G-241 dyes are available inpowder form from Nagase, Japan. The spectra of the ordinary α_(O) andextraordinary α_(E) absorption coefficients of these two dyes are shownin FIG. 15 and FIG. 16 in units of nm⁻¹ for a 1 wt. % dye mixture.

Adding isotropic dyes to the liquid crystal will shift all the colorcoordinates of the f-stop gamut by the same amount in a direction thatdepends upon the particular dye color; but since all coordinates areshifted by the same amount, the overall extent of the f-stop gamutremains the same. The same effect could be achieved by simply adding anexternal color filter. The absorption of light by anisotropic dyesdissolved in the liquid crystal, on the other hand, depends on theliquid crystal director orientation. Positive dichroic dyes can not onlyshift the f-stop color gamut, but also reduce its size. The overall sizeis decreased because, for large f-stop settings, the director fieldinside the VAN liquid crystal devices is nearly homeotropic and theincoming light interacts with the smaller, ordinary absorptioncoefficient. This shifts the color coordinates by a lesser amount thanthat which would happen for the lower f-stop settings, where the liquidcrystal is tilted away from homeotropic alignment, allowing the light toalso interact with the larger, extraordinary absorption coefficient. Apositive dichroic dye would also have a similar effect in the firstimplementation of the first embodiment, in which the liquid crystal hasa positive dielectric anisotropy.

Rather than incorporating the dichroic dye into the two liquid crystaldevices of the iris, the dichroic dye or mixture of dyes could be addedto one or more separate liquid crystal electro-optical devices calledcolor trimming devices. Because they are electrically decoupled from thetwo iris liquid crystal devices, color trimming devices can beindependently driven to minimize amount of color shift between thef-stop settings and make the iris more achromatic. FIG. 17 is a blockdiagram of the disclosed liquid crystal camera iris incorporating afirst color trimming device 56 and a second color trimming device 58located outside of polarizers 46 and 48 of the liquid crystal iris ofthe first, second, or third embodiments indicated by the dashed-linerectangle. For simplicity, liquid crystal iris 10 is shown and itselectrode structures, substrates, director fields, and retardation filmshave been omitted from the diagram. First and second liquid crystaldevices 12 and 14 of iris 10 are both driven with a voltage, V, which isadjusted to substantially produce the desired f-stop setting. First andsecond color trimming devices 56 and 58 are independently driven withcolor trimming voltages, V₁ and V₂, which in cooperation with the irisvoltage, V, are adjusted to produce the desired f-stop setting with themost least amount of color tint and color shift between f-stop numbers.Once iris 10 has been calibrated, the appropriate V, V₁, and V₂ drivevoltages, can be, for example, entered into a look-up table, making itpossible to rapidly switch between a series of achromatic f-stopsettings without requiring further adjustments.

Color trimming devices 56 and 58 are preferably ECB liquid crystaldevices that contain a dichroic dye or dye mixture added to the liquidcrystal. These color trimming devices are often referred to asguest-host ECB devices. To achieve the greatest color trimmingeffectiveness, the azimuthal direction of the surface contactingdirectors of ECB color trimming devices 56 and 58 is oriented parallelto the polarized light transmission direction of their adjacentpolarizers 46 and 48, respectively.

The following example illustrates the use of a single color trimmingdevice composed of a VAN liquid crystal device with 4.8 μm cell gap and0.2% concentration of the yellow dye G-470. For this example, the liquidcrystal devices of the iris are also VAN liquid crystal devices. FIG. 18shows the simulated u′, v′ color coordinates on the 1976 CIE uniformcolor space for unit f-stop settings varying from f/2 to f/8, comparingthe case with no color trimming device to the case with the single VANfirst color trimming device. Referring to FIG. 18, with color trimming,the color differences between low f-stop numbers are significantly lessthan those without color trimming, and the colors are much closer to thestandard white point D55. For this example, the table below shows thecolor trim voltage, V₁, and the iris drive voltage, V, required toachieve the color coordinates of the f-stop settings given in FIG. 18.

Drive voltages for first color trimming device, V₁, and iris liquidcrystal devices, V, required to achieve optimum achromatic color forf-stop settings shown. f/# V₁ (volts) V (volts) f/2 0.00 3.50 f/3 3.132.85 f/4 5.07 2.59 f/5 7.11 2.44 f/6 7.11 2.34 f/7 7.11 2.25 f/8 7.112.19

The material constants of liquid crystals, such as elastic constant,dielectric constants, and refractive indices, are known to betemperature dependent. The required voltage to achieve a given f-stopsetting will, therefore, depend upon the temperature of the liquidcrystal camera iris. FIG. 19 shows the temperature dependence of thedrive voltages required to maintain f-stop settings of f/2, f/6, andf/10 over a temperature range of 5° C. to 40° C. The data for FIG. 19were taken from measurements on the second implementation of the firstembodiment with negative dielectric anisotropy liquid crystal, althoughf-stop settings from other examples and embodiments of the disclosedliquid crystal camera iris would also be expected to show temperaturedependent drive voltages. It is possible to automatically adjust thedrive voltages for this temperature dependence by installing atemperature sensor at or near the iris location to send temperatureinformation to a control circuit that will appropriately adjust thedrive voltage. This could be accomplished, for example, with thermistorplaced at the iris and a look-up table implemented in the drive voltagecontrol circuit.

FIG. 20 shows a simulated transmitted luminance electro-optic curve ofan example of the second embodiment of the disclosed electro-opticliquid crystal camera iris. The construction of the second embodiment isthe same as that of liquid crystal iris 10 shown in FIG. 5, except asdescribed below. The director fields of the liquid crystal devices ofthe second embodiment differ from the director fields 16 and 20 of ECBliquid crystal devices 12 and 14, and from director fields 16′″ and 20′″of ECB liquid crystal devices 12′″ and 14′″, of the first embodiment.The liquid crystal devices and their associated director fields of thesecond embodiment are, therefore, indicated by corresponding referencenumerals followed by primes. The liquid crystal MLC-7030 (Δn=0.1126) isused in the simulation, and the cell gap of each of liquid crystaldevices 12′ and 14′ is set at 3.07 μm so that the product of cell gap,d, times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.629, where λ is the design wavelength of550 nm. The pretilt angle is 3°. Liquid crystal device 12′ has a layertwist angle of 60° along the cell thickness dimension from alignmentsurface 30 ₁′ to alignment surface 30 ₂′. Liquid crystal device 14′ hasa layer twist angle of −60° along the cell thickness dimension fromalignment surface 38 ₁′ to alignment surface 38 ₂′. The twist angle ofliquid crystal device 14′ is of opposite twist or rotational sense tothat of liquid crystal device 12′ because director field 20′ in liquidcrystal device 14′ is a mirror image of director field 16′ in liquidcrystal device 12′. Liquid crystal devices 12′ and 14′ are placedtogether so that azimuthal directions 42′ of surface-contactingdirectors 18 c′ and 22 c′ at the adjoining or confronting surfaces ofsubstrate plates 24 ₂′ and 32 ₁′ of the respective liquid crystaldevices 12′ and 14′ are in parallel alignment. (Because of the 60°-twistangle, azimuthal directions 42′ represent the projections ofsurface-contacting directors 18 c′ and 22 c′ on the surfaces ofsubstrate plates 24 ₂′ and 32 ₁′, respectively.) In the secondembodiment, the input polarization direction is set to approximatelybisect the angular distance between azimuthal directions 40′ and 42′ ofsurface-contacting directors 18 c at the respective alignment surfaces30 ₁′ and 30 ₂′ of liquid crystal device 12′. The normalized transmittedluminance is 50% for an applied voltage of 2.61V, and the normalizedtransmitted luminance at 6.63V is 0.1%, thereby resulting in a contrastratio of 1,000. The second embodiment requires no external retardationto achieve a high contrast ratio.

FIG. 21 shows the viewing angle dependence of the normalized transmittedluminance under application of a drive voltage of 2.61V, which produces50% normalized transmitted luminance at normal incidence. These data arepresented in FIG. 21 in the form of a normalized iso-transmittedluminance polar contour diagram. Comparing FIG. 21 with FIG. 2 for theprior art TN iris, it is apparent that there is remarkably less angularvariation in transmitted luminance. The liquid crystal iris of thesecond embodiment is eminently suitable for use as a camera iris becauseit can achieve a high contrast ratio and exhibits capability to maintaina uniform gray level over a wide range of light input angles.

FIG. 22 shows a simulated transmitted luminance electro-optic curve ofan example of a third embodiment of the disclosed electro-optic liquidcrystal camera iris. The construction of the third embodiment is thesame as that of liquid crystal iris 10 shown in FIG. 5, except asdescribed below. The director fields of the liquid crystal devices ofthe third embodiment differ from director fields 16 and 20 of ECB liquidcrystal devices 12 and 14, and from director fields 16′″ and 20′″ ofliquid crystal devices 12′″ and 14″, of the first embodiment. The liquidcrystal devices and their associated director fields of the thirdembodiment are, therefore, indicated by corresponding reference numeralsfollowed by double primes. The liquid crystal MLC-7030 (Δn=0.1126) isused in the simulation, and the cell gap of each of liquid crystaldevices 12″ and 14″ is set at 2.18 μm so that the product of cell gap,d, times the birefringence of the liquid crystal, Δn, is approximatelygiven by the formula Δn·d/λ=0.447, where λ is the design wavelength of550 nm. The pretilt angle is 3°. Liquid crystal device 12″ has a layertwist angle of 90° along the cell thickness dimension from alignmentsurface 30 ₁″ to alignment surface 30 ₂″. Liquid crystal device 14″ hasa layer twist angle of −90° along the cell thickness dimension fromalignment surface 38 ₁″ to alignment surface 38 ₂″. The twist angle ofliquid crystal device 14″ is of opposite twist sense to that of liquidcrystal device 12″ because director field 20″ of liquid crystal device14″ is a mirror image of director field 16″ of liquid crystal device12″. Liquid crystal devices 12″ and 14″ are placed together so thatazimuthal directions 42″ of surface-contacting directors 18 c″ and 22 c″at the adjoining or confronting surfaces of substrate plates 24 ₂″ and32 ₁″ of the respective liquid crystal devices 12″ and 14″ are inparallel alignment. (Because of the 90°-twist angle, azimuthaldirections 42″ represent the projections of surface-contacting directors18 c″ and 22 c″ on the surfaces of substrate plates 24 ₂″ and 32 ₁″,respectively.) In the third embodiment, the input polarization directionis set to approximately 20° with respect to azimuthal direction 40″ ofsurface-contacting directors 18 c″ at light input surface 52″ of liquidcrystal device 12″. The normalized transmitted luminance is 50% for anapplied voltage of 2.69V, and the normalized transmitted luminance at6.63V is 0.1%, thereby resulting in a contrast ratio of 1,000. The thirdembodiment requires no external retardation to achieve a high contractratio.

FIG. 23 shows the viewing angle dependence of the normalized transmittedluminance under application of a drive voltage of 2.69V, which gives 50%transmitted luminance at normal incidence. These data are presented inFIG. 23 in the form of a normalized iso-transmitted luminance polarcontour diagram. Comparing FIG. 23 with FIG. 2 for the prior art TNiris, it is apparent that there is remarkably improved uniformity of theangular dependence of the 50% gray level. The liquid crystal iris deviceof the third embodiment is eminently suitable for use as a camera irisbecause it can achieve a high contrast ratio and exhibits capability tomaintain a uniform gray level over a wide range of light input angles.

FIG. 24 is a block diagram of an example of a camera module 60 thatincludes the liquid crystal iris of any of the three embodiments of thedisclosed electro-optic liquid crystal camera iris. FIG. 24 shows liquidcrystal iris 10 of the first embodiment for purposes of convenience.Camera module 60 includes optical components of miniature sizes that canfit within a smart phone housing and provide full motion video and stillphotographs. Incoming light 50 entering camera module 60 is incident onliquid crystal iris 10, which controls the amount of light passingthrough it. A lens 62, which is illustrated as a single component butcould be a compound lens assembly, collects the light and focuses theimage onto an image sensor 64 of, for example, a CCD or CMOS type. Animage processor 66 processes the image data and sends image informationto a controller 68, which exports the image to memory, a liquid crystaldisplay screen, or other storage or display medium. Controller 68 alsosends image information to an iris driver 70, which then applies theappropriate signals to liquid crystal iris 10 to control the amount oflight reaching image sensor 64. Control signals can also be applieddirectly to iris driver 70.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles thereof. For example, opticallytransparent electrodes 26 ₁ and 26 ₂ of liquid crystal device 12 andoptically transparent electrodes 34 ₁ and 34 ₂ of liquid crystal device14 can be patterned into concentric rings to produce an adjustable depthof field. This alternative is applicable also to the other embodimentsdescribed. Moreover, for liquid crystal device 12, the value of angle αmade by surface-contacting directors 18 c with alignment surface 30 ₁need not be equal to the value of angle α made by surface-contactingdirectors 18 c with alignment surface 30 ₂; and, for liquid crystaldevice 14, the value of angle α made by surface-contacting directors 22c with alignment surface 38 ₁ need not be equal to the value of angle αmade by surface-contacting directors 22 c with alignment surface 38 ₂.This alternative is applicable also to the other embodiments described.The scope of the present invention should, therefore, be determined onlyby the following claims.

The invention claimed is:
 1. A high-contrast electro-optic liquidcrystal camera iris providing angle independent transmission of incidentlight for uniform gray shades, comprising: first and second lightpolarizing filters; a first liquid crystal device including aspaced-apart pair of first electrode structures and a second liquidcrystal device including a spaced-apart pair of second electrodestructures, the first and second liquid crystal devices including liquidcrystal material mixed with a dye to improve achromatic performance overa range of f-stop settings of the iris, and the first and second liquidcrystal devices positioned between the first and second light polarizingfilters and arranged in optical series so that a surface of one of thefirst electrode structures adjoins or confronts a surface of one of thesecond electrode structures to form an interface between the first andsecond liquid crystal devices, the interface characterized bysubstantially no selective polarization state blocking of incident lightpropagating from one to the other of the adjoining or confrontingsurfaces of the first and second electrode structures; the first liquidcrystal device having spaced-apart first alignment surfaces which areformed on interior surfaces of the first electrode structures andbetween which are confined first liquid crystal directors, the firstliquid crystal directors forming a first director field and includingfirst surface-contacting directors that contact, and define an azimuthaldirection on, each of the first alignment surfaces; the second liquidcrystal device having spaced-apart second alignment surfaces which areformed on interior surfaces of the second electrode structures andbetween which are confined second liquid crystal directors, the secondliquid crystal directors forming a second director field and includingsecond surface-contacting directors that contact, and define anazimuthal direction on, each of the second alignment surfaces; one ofthe first and second director fields being a mirror image of the otherof the first and second director fields; and the azimuthal directionsdefined on the first and second alignment surfaces formed on respectiveones of the adjoined or confronting first and second electrodestructures being in parallel alignment.
 2. The liquid crystal camerairis of claim 1, further comprising one or more external retardationfilms positioned at one or more locations that include a locationbetween the first polarizing filter and the first liquid crystal device,a location between the first and second liquid crystal devices, and alocation between the second liquid crystal device and the secondpolarizing filter.
 3. The liquid crystal camera iris of claim 1, inwhich the first and second electrode structures are patterned intoconcentric rings to produce an adjustable depth of field.
 4. The liquidcrystal camera iris of claim 1, in which the first and second lightpolarizing filters have transmission axes that are perpendicular to eachother.
 5. The liquid crystal camera iris of claim 1, in which the firstand second liquid crystal devices are homogeneously orientedelectrically controlled birefringence (ECB) devices containing liquidcrystal with positive dielectric anisotropy.
 6. The liquid crystalcamera iris of claim 1, in which the first and second liquid crystaldevices are quasi-homeotropically oriented electrically controlledbirefringence (ECB) devices containing liquid crystal with negativedielectric anisotropy.
 7. The liquid crystal iris of claim 6, in whichthe first light polarizing filter receives incoming light for successivepropagation through the first liquid crystal device and the secondliquid crystal device, and in which the second light polarizing filterreceives light exiting the second liquid crystal device, and furthercomprising an optical retarder positioned between the second liquidcrystal device and the second light polarizer.
 8. The liquid crystaliris of claim 6, in which the first light polarizing filter receivesincoming light for successive propagation through the first liquidcrystal device and the second liquid crystal device, and in which thesecond light polarizing filter receives light exiting the second liquidcrystal device, and further comprising an optical retarder positionedbetween the first liquid crystal device and the first light polarizer.9. The liquid crystal camera iris of claim 1, in which the first andsecond liquid crystal devices contain liquid crystal with positivedielectric anisotropy, and in which the first and second liquid crystaldirector fields are of opposite rotational sense.
 10. The liquid crystalcamera iris of claim 9, in which the first and second liquid crystaldirector fields of opposite rotational sense define twist angles ofabout 60°.
 11. The liquid crystal camera iris of claim 10, in which thefirst liquid crystal device has a light input face on which incominglight having an input polarization direction is incident; in which thefirst surface-contacting directors define different azimuthal directionson different ones of the spaced-apart first alignment surfaces, thedifferent azimuthal directions thereby defining an angular distancebetween them; and in which the input polarization direction bisects theangular distance between the different azimuthal directions.
 12. Theliquid crystal camera iris of claim 9, in which the first and secondliquid crystal director fields of opposite rotational sense define twistangles of about 90°.
 13. The liquid crystal camera iris of claim 12, inwhich one of the first electrode structures of the first liquid crystaldevice has a light input face on which incoming light having an inputpolarization direction is incident, and in which the firstsurface-contacting directors contacting the first alignment surfaceformed on the interior surface of the one of the first electrodestructures define an azimuthal direction that makes an angle of about20° with the input polarization direction.
 14. The liquid crystal irisof claim 1, further comprising a color trimming device to reduce anamount of color shift over a range of f-stop settings of the iris toimprove its achromaticity, the color trimming device including a liquidcrystal electro-optic device that is electrically de-coupled from thefirst and second liquid crystal devices.
 15. The liquid crystal camerairis of claim 1, in which the first and second liquid crystal devicesare electrically controlled birefringent (ECB) devices that includerespective first and second liquid crystal cells, the first and secondliquid crystal cells having cell gaps that provide voltage-dependentphase shift changes of polarization components of the incident lightpropagating through the respective first and second liquid crystaldevices, and in which the phase shift changes imparted by the first andsecond liquid crystal devices add to provide light transmittance throughthe camera iris.